So called "roof top" air conditioning systems have steadily evolved since their introduction in the 1960's for heating and cooling of commercial buildings. Roof top units are characterized as forced air units that distribute the conditioned air (either heated or cooled) by means of fans, through supply ducts, to each of the ventilated zones of a building. The systems are semi-closed loop systems in the sense that conditioned air is circulated to the zones of a building to cool or heat the zones, and then is returned by return ducts to the heating or refrigeration system to be heated or cooled and recirculated to the zones. Outdoor ambient air may be admitted for cooling purposes or to maintain an adequate indoor air quality, as will be further explained.
The most recent roof top designs are variable air volume (VAV) systems. VAV systems are designed to operate at a constant supply air temperature, for example, 55.degree. F. The volume of the supply air provided to the ventilated zones of the building is varied in order to satisfy particular cooling or heating requirements, but the temperature of the air is maintained constant. On a hot day or when the zones are fully occupied by people, a high volume of cooled air at 55.degree. F. would be needed to satisfy the cooling requirements. On a cool day or when few people are occupying the building, a substantially reduced volume of cooled air at 55.degree. would be required to meet design cooling requirements. The operating speeds of fans in a VAV system are varied to vary the volume of conditioned air that is being supplied to the various zones at any given time, thereby keeping the temperature of the zones at a desired setpoint temperature of, for example, 72.degree. F. as sensed by the thermostat in the zone. The 55.degree. supply air temperature and the sensed temperature in the zone become controlling factors for the VAV system.
The first VAV air conditioning systems provided all cooling by mechanical means. Thus, the compressor and the heat exchange coil were required to be operated frequently in order to cool the air distributed throughout a building. This was true even when the outside ambient air was relatively cool. The compressors require relatively large amounts of energy for operation. As ways were sought to improve the efficiency and reduce the cost of cooling air, economizers were designed and installed on air conditioning systems. An economizer is a device that introduces outside air into the system to provide cooling when the ambient air has an energy level that makes this possible. Since VAV systems always supply air at a constant temperature of nominally 55.degree. F., outside air is generally effective to assist in cooling at all times that the outside air temperature is less than 55.degree. F.
Outside air is mixed by an economizer in a VAV System with the return air from the building cooling zones. The outside air is provided by the fans as cooled supply air to the ventilated zones. It will be appreciated that, as the temperature of the outside air is reduced, a much reduced volume of outside air is needed to cool the zones to a desired temperature. When the outside air is, for example, 10.degree. F., very little outside air needs to be added to the return air to reduce the return air to 55.degree. F. and to keep the zones at 72.degree. F. In such conditions, the total air flow through the air conditioning system can typically be less than one third of the air conditioning system's full capacity, which occurs during 100% mechanical cooling.
The reduced volume of air flow at the lower outside air temperatures results in a problem called stratification in the supply ducts. This simply means that the return air and the outside air are not adequately mixed together prior to entry into the supply ducts. When the air in the main supply duct descending into the building becomes stratified, one side of the duct will have relatively warm return air in it while the other side of the duct has relatively much cooler outside air in it.
The problem of stratification is made worse in the more recent VAV systems that are of a side-by-side' design. Side-by-side VAV systems are designed to have return air enter the rooftop unit on one side of the unit and the outside air enter on the other side, as opposed to the "over and under" orientation of previous designs. The return air and the outside air each pass through a damper assembly and enter a common plenum, where mixing should occur. Side-by-side designs are preferable in that the air that is directed onto the coils of the evaporator is more uniform in temperature from the top to the bottom of the unit. The side-by-side design corrects some freezing problems experienced in the heat exchange coil with over-and-under designs. However, at low air flow rates, the side-by-side design results in the fans drawing the return air and the outside air through the plenum of the rooftop unit in unmixed parallel, side-by-side flows. The air then enters the descending supply ducts in a stratified, unmixed manner. The problem of stratification is even further exacerbated as rooftop designs are made ever wider in order to add additional cooling capacity. Current VAV systems may be between four and ten feet wide. The plenum necessarily becomes wider, making it more difficult to effect adequate mixing under low flow conditions.
As previously indicated, the temperature of the supply air as the supply air enters the main supply duct is an important controlling parameter for the VAV system. Measurement of the temperature is typically accomplished by means of a thermistor. The output of a thermistor is a resistance that varies with temperature. In order to obtain an average supply air temperature, a grid including a plurality of thermistors has been constructed utilizing pairs of series and parallel connected thermistors.
In order to obtain an output from the above described grid that has the same characteristics as a single thermistor, an equal number of series pairs of thermistors and parallel pairs of thermistors must be coupled together to obtain the desired averaging. Accordingly, the system may employ a single thermistor, or grids of four, eight, sixteen, etc. thermistors as desired to obtain the average temperature.
Obviously, the more thermistors in the grid that are utilized, the more accurate is the average output. Such grids have been mounted on a lattice formed of rods that are welded together and suspended over the mouth of the supply air duct. This method of mounting is rather complex and expensive to produce and install. Additionally, and more troublesome, is the fact that the failure of a single thermistor in a grid results in an open circuit, completely invalidating the output of the grid. To correct such a failure, the VAV system must be shut down and maintenance personnel must enter the VAV system enclosure and individually test each thermistor in the grid in order to determine which thermistor has failed.
It is a primary object of the present invention to provide an accurate average temperature sensor that is effective throughout the full range of air flow rates delivered by the VAV system to the supply air duct.
It is a further object of the present invention to minimize the complexity of the installation and maintenance of such sensor while retaining accurate temperature averaging.
It is another object of the present invention to provide a condition sensor arranged between a high and low pressure region such that a flow of air is passed over the sensor.
These and further objects of the present invention will become apparent from the following description of the preferred and alternate embodiments.